linux-kernel - Re: futex() on vdso makes process unkillable

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Message-Id: <20100126113312.5AA3.A69D9226@jp.fujitsu.com>
Date:	Tue, 26 Jan 2010 11:41:05 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Darren Hart <dvhltc@...ibm.com>
Cc:	kosaki.motohiro@...fujitsu.com,
	Peter Zijlstra <peterz@...radead.org>,
	Mark Seaborn <mrs@...hic-beasts.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"hugh.dickins" <hugh.dickins@...cali.co.uk>
Subject: Re: futex() on vdso makes process unkillable

> Peter Zijlstra wrote:
> > On Mon, 2010-01-25 at 16:27 +0900, KOSAKI Motohiro wrote:
> <snip>
> >> Futex should work both file anon anon. however I personally think 
> >> vdso is not file nor anon. it is special mappings. nobody defined
> >> futex spec on special mappings. (yes, undefined).
> >>
> >> Personally, I think EINVAL or EFAULT are best result of vdso futexing, like as
> >> futexing againt kernel address. but I guess another person have another thinking.
> >>
> >> I'd like to hear futex folks's opinion.
> > 
> > Well, my opinion is we should remove the vdso, its ugly as hell :-)
> > 
> > But I think it would make most sense to extend its definition in the
> > direction of it being a file (for all intents and purposes its a special
> > DSO -- which unfortunately isn't present in any filesystem).
> > 
> > [ For all intents and purposes processes can already communicate through
> > futexes on the libc space, so being able to do so through the vsdo
> > really doesn't add anything ]
> > 
> > So the problem is that the VDSO pages do not have a page->mapping
> > because they lack the actual filesystem part of files, so even if (with
> > the recent zero-page patch from Kosaki-san) you make private COWs of the
> > VDSO, you'll get stuck in that loop.
> > 
> > So the prettiest solution is to simply place the vdso in an actual
> > filesystem and slowly migrate towards letting userspace map it as a
> > regular DSO -- /sys/lib{32,64}/libkernel.so like.
> > 
> > [ that has the bonus of getting rid of install_special_mapping() ]
> > 
> > The ugly solution is special casing the vdso in get_futex_key().
> 
> I like the creating-a-real-file solution. However, for now (and for 
> stable), I think Kosaki's suggestion of EINVAL or EFAULT is a good 
> stop-gap. EINVAL might play the best with existing glibc implementations.

May I confirm your mention?

If we can accept EFAULT, we don't need any change. my previous futex patch
already did. because 1) VDSO is alwasys read-only mapped 2) write mode
get_user_pages_fast() against read-only pte/vma return EFAULT.

Current linus and stable tree don't cause Mark's original problem. instead, just
return EFAULT. (Well, I'm sorry. my previous mail was unclear. I wrote v2.6.31 test
result)

If you can't accept EFAULT, we need to add vdso specific logic into get_futex_key().
Is this your intention?



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